Catalyst systems which have been proposed for this process, include catalysts based on soluble Group VIII metal compounds, for example of cobalt or rhodium.
Aldehyde precursors for use in this process may, for instance, be obtained by hydroformylation of an olefinically unsaturated compound in the presence of a Group VIII metal catalyst, which process has attained industrial application and is also known as the oxo process. Frequently, the aldehyde produced by hydroformylation of an olefin is separated from the reaction mixture obtained by the hydroformylation to eliminate the catalyst and by-products and is subsequently used in the hydrogenation.
U.S. Pat. No. 4,263,449 discloses a process for the preparation of alcohols, wherein the aldehyde-containing reaction product of a hydroformylation reaction is used as such in a subsequent hydrogenation reaction catalyzed by a heterogeneous Raney nickel or cobalt catalyst. Water is added for generating a biphasic reaction product facilitating separation of the catalysts used. Apart from the inherent complications of the use of a plurality of catalysts, the use of active Raney catalysts will concurrently hydrogenate any olefin values remaining in the hydroformylation product.
According to Great Britain Patent No. 1,270,985, cobalt carbonyls modified by tertiary phosphines, known to be active as hydroformylation catalyst, can be used in the hydrogenation of aldehydes to alcohols under an atmosphere comprising both hydrogen and carbon monoxide. However, high reaction temperatures and pressures are required for this process.
Other known hydrogenation processes require a pure hydrogenation atmosphere, so that carbon monoxide should be removed, if an aldehyde-containing hydroformylation product is to be directly used.
A catalytic system comprising a compound of palladium and a bidentate phosphine is described by Y. Ben David et al., in J. Am. Chem. Soc., 1989, 111, 8742-4, but only for use in the carbonylation of aryl chlorides.
Therefore, there remains a continued need for improved and more versatile catalysts for the hydrogenation of aldehydes.